You might want to change the extraction line so that your code still works for programs over 100MB.

sub refresh{
      my @size = split "\n", `cat /proc/$pid/status`;
      (my $vmsize) = grep {/VmSize/} @size;
      my ($size)   = $vmsize =~ /VmSize:\s+(.*)$/;   # Changed this
      $t->configure(-text=>"PID: $pid ->  $size");
      if($size eq ''){Tk::exit}
}
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